WG15 Defect Report Ref: 9945-1-amd1-03 Topic: mmap

This is an approved interpretation of 9945-1-amd1-1993.

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Last update: 1997-05-20

9945-1-amd1-93 #3
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Defect Report Number: (to be assigned by WG15)
Topic: mmap
Relevant Sections: 12.2.1.2
Classification: (to be assigned)
Defect Report:
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Date: Mon, 10 Oct 94 9:15:18 PDT
From: Kurt Peterson (kurtp@cup.hp.com)
I'd like to receive clarification on text I read in 9945-1-amd1-1993.
Specifically the question has to do with mmap() and what is meant by the
following(removed word for word from page 236, lines 213-215):
"The mapping established by mmap() replaces any previous mappings for
those whole pages containing any part of the process's address space
starting at "pa" and continuing for "len" bytes."
There are a couple of ways this can be interpreted and I'd like to find out
which is the intended one.
a) Does this mean if I map page 3 of a file in one call and page 5 in
another call and then map pages 3-5 in a separate call, the call
succeeds and the previous mappings of page 3 and page 5 are no
longer valid? If so, does that apply to both MAP_PRIVATE and
MAP_SHARED?
b) Does this apply to MAP_FIXED where lets say the user has page 1 of
file A mapped at address 0x40005000 and then a mmap(MAP_FIXED) at
address 0x40004000 for three pages(range 0x40004000-0x40006fff) on
file B overwrites the other mmap? In other words am I allowed
overlay mappings with different files? If so, both MAP_SHARED
and MAP_FIXED?
c) Does the overwrite only apply to mmap() collisions or any address
collision. For instance, can a process mmap() over its data object
or other non mmap objects?
WG15 response for 9945-1-amd1-1993
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For question a), the standard is clear that the mapping is replaced the
addresses overlap but not replaced where they don't. Page 238 line
274: "...nor shall it replace an extant mapping."
Part one of the question: ... the other mappings are no longer valid? ->
this is incorrect, unless the addresses are specified in MAP_FIXED and
overlap. The standard is clear that this applies to
both MAP_PRIVATE and MAP_SHARED.
Question b, the standard is clear that you can replace the existing mappings
using MAP_FIXED independent of MAP_SHARED and MAP_PRIVATE.
Question c, the standard is silent on whether you can map over objects other
than ones mapped by MMAP. Implementations can allow MAP_FIXED mapping over
other kinds of memory or not allow it.
Rationale
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None.